Almost any type of material which can be twisted, pulled, extruded, spun, stretched, or otherwise fabricated into a filament or fiber can be used to make ropes. Basically, a rope is an elongate structural element which is fabricated from any collection of elongated members, such as filaments or fibers, which are manufactured into some type of a long, structural line which is relatively flexible and capable of carrying tensile loads.
Herein, the term “rope” refers to rope, cord, wire rope, cable, and the like.
Herein, the term “webbing” refers to fibrous tension members which are substantially flat and comprised of fibers woven, bundled, knit, braided, felted, or twisted together. Webbing includes strong, narrow, closely woven fabric used especially for seat belts and harnesses or in upholstery.
Herein, the term “fibrous tension member” refers to rope or webbing comprising multiple threads woven, bundled, knit, braided, felted, or twisted-together such that the resultant member is at least somewhat flexible.
Elongation, stress, and strain are generally related to each other. For example, if a rope supporting a load elongates one inch and is operating in its elastic range, the strain is also one inch and the stress may be deduced by knowing the length of rope being loaded, its spring constant, and knowing whether elongation is increasing or decreasing (hysteresis). If one tracks elongation over time, one knows which hysteresis curve should be used to relate elongation to stress. Also, if one tracks elongation over time, one can distinguish non-recoverable plastic deformation (yield) from elastic strain. For these reasons, for the purposes of this application in both the specification and the claims, the term “elongation” refers to elongation, stress, or strain.
Most common ropes are manufactured by the following process:                1. Relatively short to moderately long filaments or fibers are twisted into yarns.        2. Yarns are twisted into cords.        3. Cords are twisted into strands. This process is called “forming.” Sometimes, extra cords, yarns, and/or filaments (made from relatively flexible materials) are added during the forming process for internal lubrication in each strand. These extra cords, yarns, and/or filaments are commonly used during the fabrication of ropes that are subjected to relatively high flexural loads.        4. Two or more strands are twisted into a rope. This process is called “laying.” Similar to Step 3, extra strands, cords, yarns, and/or filaments (made from relatively flexible materials) can be added during the laying process to improve internal lubrication in the rope.        5. Two or more ropes are twisted into a wire rope or cable. Similar to Step 4, extra elongated members can be added to improve internal lubrication in the cable.        
Ropes may alternatively be manufactured using bundling, weaving, and/or felting techniques. Many ropes have external materials applied to the yarns, cords, or strands to improve environmental resistance, as well as handling characteristics. Application processes for these materials include galvanizing, bonding, painting, and coating.
Ropes and webbing are integral to a wide range of activities. The potential cost in equipment damage, personnel injuries and even lives of failing or overloaded ropes is high. The fiscal cost of maintaining and inspecting ropes and webbing is high. Safety factors in ropes and webbing are significant, on order five to fifteen times expected load, with inherent weight cost.
An external load sensing element such as a load cell can be used to measure stress on a rope. This provides stress measurement at a point such as a pulley connection or the interface between the rope and a load. However, sometimes the elongation varies along the rope which would not be discernable with a point measurement such as that provided by a load cell. In addition, some applications such as rock climbing, would not easily allow the permanent connection of a load cell to a rope so the rope may be used when it is not monitored, allowing damage to occur without monitoring.
Various means have been proposed for providing an indication of damage to ropes and webs. In U.S. Pat. No. 5,834,942 to Pethrick et. al., a synthetic fiber cable is disclosed which includes one or more electrically conductive indicator threads placed into the strands to monitor the state of the cable. A tearing of the fiber may be detected by applying a voltage to the indicator thread. In this manner, each individual strand of a synthetic fiber cable can be checked and the cable can be replaced when a predetermined number of torn strands have been exceeded.
In the case of the above-mentioned patent, the indicator threads and sensing unit are capable of detecting when a threshold voltage limit value is exceeded by torn indicator threads. The Pethrick system particularly shows a threshold value switch SW to binarize the output and their discussion speaks only of setting this threshold value to that which would indicate breakage of the indicator thread.
In the case of the above-mentioned patent, the indicator threads connect to the sensing unit via connecting elements—physical contacts at the end of the cable. This limits the application to cases where the end of the cable is accessible to the sensing unit and the data produced refers to the cable's entire length as there is no provision for sensing a portion of the cable.
Various means have been proposed for providing a measure of strains and kinks in ropes. In U.S. Pat. No. 5,182,779 to D'Agostino et al., a rope is disclosed which includes one or more optical fibers placed into the strands to monitor the state of the rope. Such a system is capable of measuring strain in the rope by means of detecting Rayleigh reflections due to density fluctuations. Such a system can detect macrobends and microbends which change the angle at which light strikes the interface between core and clad, causing light to be absorbed into the clad or reflected back to the source. Such a system can use optical time domain reflectometry (OTDR) to detect and locate breaks resulting in Fresnel reflections. Such a system can use preformed optical fiber to minimize residual stresses in the indicator fiber resulting from twisting in the rope manufacturing process. Preforming is the process of twisting an elongated member, such as a filament in the opposite direction as the twisting process to make a rope so the indicator thread is relatively untwisted in the final rope. Such a system can use prestressed rope to allow the rope to strain past the breaking point of the optical indicator fiber.
Such a system requires a sophisticated optical sensing-processing unit. Accordingly, there is a need in the art for an improved system and method for measuring elongation or curvature experienced globally or locally by fibrous tension members.